1. Field of the Invention
The present invention relates to a radio communications system, a base station for a radio communications system, and an intermittent power-on type mobile station each of which is particularly suitable for in radio local area networks (LANs).
2. Description of the Related Art
FIG. 21 is a block diagram showing a communications model in which a radio LAN is applied as a general radio communications system. Referring to FIG. 21, the radio LAN 100 is formed of a plurality of radio terminals 102 and a base station 101 accommodating radio terminals 102 covered in a radio area 103. Communications is established between two radio terminals 102 by connecting via a radio network in a cableless state.
The base station 101 in each of a plurality of radio LANs 100 is connected to the cable network 104 accommodating cable terminals 105. The radio terminals 102 can communicate with cable network 104 or terminals (radio terminals 102 or cable terminals 105) on the radio network via the base station 101.
The base station 101 emanates regularly a synchronous beacon signal to a radio terminal 102 to synchronize with the radio terminal 102 and to control the same.
The radio terminal 102 can be used as a portable, mobile terminal. However, it is required that the power consumption of the radio terminal 102 acting as a mobile terminal reduced.
The general radio LAN 100 shown in FIG. 21 has a control mode in which the mobile station (radio terminal) 102 is intermittently powered on under the control of the base station 101 to achieve its power-saving operation.
That is, data transmission information (terminal information to a transmission destination) which is transmitted to a radio terminal (PS Power Save) station) 102 to be operated in a power-saving mode is added regularly to a specific one among beacon signals emanated from the base station 101.
For example, as shown with the timechart in FIG. 22, the base station 101 outputs beacon signals (refer to symbols ".largecircle." and ".circleincircle.") at predetermined intervals (refer to the time (t1), (t2), . . . , (t5). Data transmission information which is transmitted to the mobile terminal 102 controlled to operate in a power-saving mode is added to the beacon signal (refer to ".circleincircle.") output every third interval among the above-mentioned beacon signals (refer to the time (t1) and (t4)).
In the mobile station 102 acting as a transmission destination terminal which receives a power-saving control beacon signal from the base station 101, the period during which the next beacon signal is received is used as a data receive-ready period (receivable period) from the base station 101 and is controlled so as to be powered on only the period (refer to the time (t1) and (t2) and the time (t4) and (t5)).
The power-saving control beacon signal (e.g., at the time (t1)) includes timing information regarding the power-saving control beacon signal to be transmitted next. The radio unit 102 acting as a PS station is powered on so as to receive the next power-saving control beacon signal (at the time (t4)), based on the timing information.
That is, the radio unit 102 acting as a PS station is powered on at the timing when the power-saving control beacon signal can be received. On the other hand, when there is data for a self station, the data is received based on the data transmission information included in a beacon signal. When there is no data for a self station, the power consumption is saved by turning off the power supply in the period during which no data is received.
The radio terminal (PS station) 102 corresponding to data transmission information included in a beacon signal (referred to the symbol ".circleincircle.") from the base station 101 is controlled to realize the power-saving operation (hereinafter the beacon signal represented with the symbol ".circleincircle." is referred to as a power-saving control beacon signal).
The base station 101 is in a data transmit-ready state to the radio terminal (CA (Continuous Active) station) 102 which does not operate in a power-saving mode during the period between the time a beacon signal (symbol ".largecircle.") for no power-saving control is output and the time the next beacon signal is transmitted (the time (t2) to (t4). At the same time, the mobile terminal 102 which operates in a power-saving mode is powered off, whereby no data is received.
Referring to FIG. 22, numerals represent the order in which transmission requests occur and arrows show data flows (mobile station acting as destination). Data are transmitted basically in numerical order (the order in which a transmission request occurs).
Data is transmitted to the radio terminal (PS station) 102 only during the data receive-ready period of the radio terminal (PS station) 102. Hence even if a transmission request occurs early, data beyond the limit in a period is forwarded to the next transmission timing. For that reason, transmission data to be transmitted to the intermittent power-on type mobile station 13 is buffered to memories (refers to numerals 33 and 34) in the base station 101 and becomes a transmission waiting state.
In such a configuration, the base station 101 powers on the mobile terminal 102 acting as a PS station only during the data transmission period (refer to the time (t1) and (t2) and the time (t4) to (t5), synchronously with the beacon signal which is emanated from the base station 101 accommodating a radio area corresponding to the position of the mobile station 102 itself. This operation allows checking the information regarding the presence or absence of transmission data sent to the self station included in a received beacon signal.
The mobile station 102 receives data for the self station. However, if there is no data for the self station, the power consumption can be reduced by turning off the power supply until the next predetermined receiving time.
For example, according to the data transmission information included in the beacon signal received at the time (t1) shown in FIG. 22, the radio terminal (PS1) 102 acting as the first PS station receives data "1" and the radio terminal (PS2) 102 acting as the second PS station receives data "2".
Similarly, according to data transmission information included in the beacon signal received at the time (t4), the radio terminal (PS1) 102 receives "8" and the radio terminal (PS2) 102 acting as the second PS station receives "5").
In the timechart shown in FIG. 22, numeral information attached to the data transmitted from the base station 101 to the mobile station 102 represents the order of a transmission request in the base station 101. Data is transmitted according to the numeral information other (the transmission data "5" and "8" are data for a PS station and are delayed compared with data "6", "7", . . . "11" for CA stations (CA1 and CA2) transmitted at the time (t2) to (t4)).
However, in the radio LAN acting as the above-mentioned radio communications system, the number of mobile stations 102 each acting as a PS station among the mobile terminals 102 connected to the base station 101 and the number of CA stations are indefinite. Moreover, the amount of data transmission and the timing to the mobile terminal 102 from the base station 101 are indefinite. There is the problem in that the method in which the mobile terminal 102 allocates the period during which data can be received from the base station 101 according to beacon signals sent at constant intervals cannot be dealt with in the above-mentioned manner.
That is, generally, where the interval during which the PS station can receive the power-saving operational beacon signals is set narrowly to improve the flow of transmission data and the throughput, the frequency at which the mobile terminal 102 acting as a PS station is powered on becomes large, whereby the power consumption on the side of the mobile station 102 is increased.
When the interval at which the above-described power-saving operational beacon signal is transmitted is set widely to reduce the power consumption, the flow of transmission data is reduced on the side of the base station 101, whereby the throughput is reduced.
Moreover, as shown in FIG. 23, when up-stream data "A" transmitted from the mobile station 102 to the base station 101 occurs in the receive-ready period (refer to the time (s1) and (s2) during which the mobile station 102 acting as a PS station can receive data signals from the base station 101 after a transmission of the power-saving control beacon signal at the time (s1), data (data "3") to be transmitted cannot be received in the receive-ready period. As a result, there is the problem in that the throughput is decreased.